[TIR] Create a StringImm reference type (apache#4806)

This is motivated by the want to send an
array of strings across the python/C++
boundary. Arrays only support ObjectRef types
and so can't carry StringImmNodes. This creates
a string reference type, StringImm, which can
be used with tvm::Arrays.

Change-Id: I598a44536c156b97dbfe3e9518e0a1f705da850c

Author: mbaret

include/tvm/tir/expr.h

@@ -337,6 +337,11 @@ class StringImmNode : public PrimExprNode {
   TVM_DECLARE_FINAL_OBJECT_INFO(StringImmNode, PrimExprNode);
 };
 
+class StringImm : public PrimExpr {
+ public:
+  TVM_DEFINE_OBJECT_REF_METHODS(StringImm, PrimExpr, StringImmNode);
+};
+
 /*!
  * \brief Cast value from one data type to another.
  * \note The lanes of value should keep fixed.